1. Technical Field
The present invention relates generally to lateral field effect transistors (FETS) with vertical channels, and in particular, to such transistors formed in wide bandgap semiconductor materials. This invention also relates to monolithic integrated circuits comprising these transistors.
2. Background of the Technology
Wide bandgap semiconductor materials (with EG>2 eV) such as silicon carbide (SiC) or Group III nitride compound semiconductors (e.g., gallium nitride GaN) are very attractive for use in high-power, high-temperature, and/or radiation resistant electronics. SiC power rectifiers and RF transistors are now commercially available, and SiC power switches as well as GaN microwave transistors are expected to appear in the commercial market in the near future.
Because of the fundamental differences in material properties and processing technologies, traditional Si or GaAs integrated circuit (IC) technologies such as Complementary Metal-Oxide-Semiconductor (CMOS) or Direct Coupled FET Logic (DCFL) cannot in most cases be easily transferred to wide bandgap semiconductor industry. Several attempts at fabricating SiC NMOS and CMOS digital and analog ICs have been reported in the last decade (e.g., [1], [2]). A monolithic CMOS integrated device in SiC and method of fabricating the same have been patented in 2002 [3]. Moreover, recent development of SiC Lateral DMOS Field-Effect Transistors (LDMOSFETs) (e.g., [4], [5]) theoretically allows for the monolithic integration of MOSFET-based control circuitry and power switch for use in Smart Power electronics. However, certain issues limit the use of MOSFET-based SiC integrated circuits in the applications where high temperature and/or radiation tolerance is required. The first such issue is on-state insulator reliability due to much smaller conduction band offset of SiC to SiO2 as compared to that of silicon [6], [7]. This issue becomes even more significant at high temperature and extreme radiation environment. Other issues include: low inversion channel mobility due to high interface state density at the SiC/SiO2 interface and high fixed charge density in the insulator; and significant threshold voltage shift with temperature due to ionization of interface states.
Another transistor candidate for the use in SiC ICs is a Metal Semiconductor Field-Effect Transistor (MESFET). Although SiC MESFET monolithic microwave integrated circuits (MMICS) have received significant attention in the last decade (e.g., [8]), there have been few published attempts to build SiC MESFET logic and analog circuits (e.g., [9]).
An alternative to the MOSFET and MESFET approaches is the use of lateral JFET-based ICs. An example of a vertical channel JFET employing a recessed gate structure can be found in U.S. Pat. No. 4,587,712 [10]. An example of a lateral JFET formed in SiC can be found in U.S. Pat. No. 5,264,713 [11]. Enhanced-mode JFET digital ICs with resistive load have also been reported [12]. JFET based ICs can also be implemented in either complementary (n-type and p-type channels as disclosed in U.S. Pat. No. 6,503,782 [13] or enhanced-depletion (n-type channels) forms. SiC JFETs have proven to be radiation tolerant while demonstrating very insignificant threshold voltage shift with temperature [14]. A major deficiency of this approach has been difficulties achieving monolithic integration of low voltage control circuitry with power switches for Smart Power electronics.